158 ECHINODERMATA— PELMATOZOA phylum iv 



brachials surrounding the peripher^ of the tegmen, either in an almost continuous 

 ring, or in groups where the interoral sutures meet the radials. Whether there are 

 further brachials in succession is unJcnown. Stern with a small axial canal. 

 Silurian and Devonian. Relations doubtful, may be monocyclic Crinoids. 



Zophocrinus S. A. Miller, has arm plates in Clusters of about three. Surface 

 smooth. Silurian ; North America. 



Tiaracrinus Schnitze {Staurosoma Barrande). Arm plates eight or ten to 

 each radius, forming a rather continuous ring. Surface strongly marked with 

 folds crossing the sutures, which seem to be accompanied by pores. Devonian ; 

 Eifel, France, Bohemia. 



Order 5. BDRIOASTEROIDBA Billings. 



(Syn. : Thyroidea Chapman, 1860; Agelacrinoidea^. K.MiWqv, 1877-1883; 

 Gystasteroidea ^tQmm.B:im^ 1888; Bernard, 1893; Thecoidea 3 a^okeX, 1895). 



The eminent Canadian paleontologist, E. Billings, as early as 1854, called 

 attention to the great difFerence between the forms now grouped under this 

 name and the typical Cystideans, and in 1858 suggested that they should be 

 arranged as a suborder to be called Edrioasteridae. Subsequent authors have 

 generally agreed to this in principle, but not as to the relative rank which the 

 group should have. Bather, regarding it as a class, assigns it equal rank with 

 the Cystids, Crinoids and Blastoids. Jaekel recognises it as one of three 

 Orders into which he divides the Cystidea {sensu L. von Buch), and this pro- 

 cedure is in principle here adopted, without, however, denying that it may be 

 entitled to the higher rank. Bather's definition and general characterization 

 of the group is substantially as follows : 



Pelmatozoa in which the theca is composed of an indefinite numher of irregulär 

 plates, some of which are variously differentiated in different genera ; with no subvective 

 skeletal appendages, but with central mouth, from which there radiate through the theca 

 five unbranched ambulacra, composed of a double series of alternating plates {covering- 

 plates), sometimes supported hy an outer series of larger alternating plates {side-plates 

 or flooring -plates). Pores between {not through) the ambulacral elements, or between 

 them and the thecal plates, permitted the passage of extensions from the perradial 

 water-vessels. Anus in posterior interradius on oral surface, closed by valvulär 

 pyramid. Hydropore {usually, if not always, present) between mouth and anus. 



This would represent primitively, as Bather explains, a form with flexible 

 sack-like calyx, composed of numerous irregulär, polygonal plates deposited 

 in the integument ; having a mouth in the centre of the upper surface, and 

 being attached by some indefinite portion of the lower surface. The struc- 

 ture of the ambulacra would remove it far from the earlier Amphoridea, 

 among Cystids, from which group it may haVe been derived. 



Upon this primitive ancestral form the following characters were, to a 

 greater or less degree, impressed : a sessile habit ; the consequent assumption 

 of a circular, flattened form; the difFerentiation of the upper and under 

 surfaces ; the development of marginals or concentric frame-plates ; and the 

 tendency to increase the food-gathering surface by spiral coiling of the ambu- 

 lacra. According to the varying extent of these modifications, the order is 

 divisible into three families : Agelacrinidae, Cyathocystidae, Edrioasteridae. 



The Edrioasteroidea have a somewhat greater geological ränge than the 



